                1. Technical Field        
Various embodiments generally relate to a memory device, and, more particularly, to a memory device including a memory cell array of a three-dimensional structure.                2. Related Art        
A semiconductor memory device is a memory device which is realized using a semiconductor such as silicon (Si), germanium (Ge), gallium arsenide (GaAs) and indium phosphide (InP). Semiconductor memory devices are generally classified into a volatile memory device and a nonvolatile memory device.
A volatile memory device is a memory device which loses stored data when power supply is interrupted. A volatile memory device includes a static RAM (SRAM), a dynamic RAM (DRAM) and a synchronous DRAM (SDRAM). A nonvolatile memory device is a memory device which retains stored data even when power supply is interrupted. A nonvolatile memory device includes a flash memory device, a read only memory (ROM), a programmable ROM (PROM), an electrically programmable ROM (EPROM), an electrically erasable and programmable ROM (EEPROM) and a resistive memory device (for example, a phase change RAM (PRAM), a ferroelectric RAM (FRAM) and a resistive RAM (RRAM).
In order to meet excellent performance and low cost that consumers demand, the degree of integration of a nonvolatile memory device is increasing. In the case of a two-dimensional or planar memory device, the degree of integration is determined by the area occupied by a unit memory cell. Therefore, recently, a memory device of a three-dimensional structure in which unit memory cells are disposed in the vertical direction has been developed.